FIG. 1 shows the functional reference configuration for conventional interworking between a DECT (digital European cordless telephone) system and a GSM (global system for mobiles) system. FIG. 1 shows that the DECT portable part (PP)1 communicates with the DECT fixed part (FP) 2 over an air interface 3, The fixed part communicates with a GSM mobile switch centre (MSC) 4 over the standard GSM A-interface 5. In the signalling plane of the A-interface mapping takes place only on the CC/NWK and MM/NWK levels, so the DECT system does not use the lower layers of the A-interface. Documents [12] to [18] listed below provide the detailed definitions of how GSM services are provided over the DECT air interface.
Document [14] in particular defines the implementation of bearer services. This takes as its basis that in the DECT/GSM interface the GSM error correcting protocol RLP is terminated to the interworking unit (IWU) of the DECT fixed part, and the DECT air interface (based on the C data profile) provides its own error correction over the DECT air interface by the re-transmitting MAC layer protocol and by the DLC layer re-transmitting the LAPU protocol. This protocol model is illustrated in FIG. 2, where block 6 represents the DECT portable part, block 7 represents the DECT fixed part and block 8 represents the GSM mobile switch centre interworking unit. Thus, in this standard implementation the RLP and LAPU protocols function independently: the RLP protocol operating over the A-interface (9 In FIG. 2) between GSM and DECT and the LAPU protocol operating over the DECT air interface (10 In FIG. 2). Details of the DECT C data profile are given in document [21] below. This defines the provision of data rates up to and including 9600 bps (bits per second). In a conventional DECT/GSM system this data speed is the limit of the data service--in which the system carries signals representing data that has been provided to the system in a digital form by a user of the DECT portable part, or that is to be provided by the system to that user in a digital form.
This solution (where the RLP protocol is run over the A-interface between the DECT fixed part interworking unit and the GSM mobile switch centre) was chosen because of standardisation demands: the standard GSM A-interface was not to be altered and the DECT/GSM interworking system had to adapt to the solutions that had already been adopted for GSM. However, one major problem of this DECT/GSM interworking arrangement is that it is limited by the relatively low data transmission rate (9600 bps) available over the GSM air interface, although theoretically the DECT system, with higher capacity over its radio interface, could provide data rates that are much higher as well as being infrastructure-compatible. Because of this, when the DECT system is transmitting data according to the model of FIG. 2 its transmission rate must be adjusted before the interface with the GSM network so as not to exceed 9600 bps. This rate is now deemed to be relatively slow--for example current modem services are capable of operating at at least 28800 bps.
Another problem with the system is that because the RLP protocol and GSM specific V.110 rate adaptation are used over the A-interface but not over the DECT air interface the DECT fixed part interworking unit has to be complicated, and less efficient than it might otherwise be. The RLP protocol was originally designed for error correction over the GSM air interface and the modified GSM V.110 rate adaptation used under the RLP protocol was designed with the GSM air interface rates (of no more than 9600 bps) in mind. In fact, these protocols are not really needed over the A-interface in DECT/GSM interworking because the DECT protocols handle the air interface link.
Another problem is that the system of FIG. 2 cannot convey modem signals because of the limitation of the A-interface. In the GSM system this type of digital data transmission was selected for data services because the GSM voice codec is not capable of conveying modem signals. However, the air interface of the DECT system can support PCM codecs and can therefore support modem signalling. If this type of signalling could be supported over the DECT/GSM interworking link it would give the user much more flexibility.
Another issue is that GSM modem services are based on the GSM network behaving as an extended V.24 link, so that in a modem connection over GSM the data termination equipment (DTE) is connected to the mobile telephone and the data connection equipment (DCE) to the mobile switch centre. In this arrangement the mobile switch centre modem (DCE) has also to establish a connection to the far end DCE (usually located in a network external to the GSM system). This can take a long time. If in a DECT/GSM interworking system it were possible to bypass the GSM V.24 interface, and even the mobile switch centre modem, the data rates could be increased and the time to establish a connection could be decreased.
The present invention aims to address some or all of these problems.